Mössbauer resonance studies in magnetically diluted Eu–Ca hexammines

Abstract

The Mössbauer effect in ¹⁵¹Eu is used to examine the mixed hexammines Eu_x Ca_1−x(NH₃)₆, with x = 0.012 and 0.092, and comparison is made to the apparent lack of conduction electron effects on the europium ionic and nuclear properties in Eu(NH₃)₆. In zero external magnetic field, very broad lines are seen, indicative of weakened dipolar spin relaxation. For the 1.2% Eu sample, the single Lorentzian linewidth at 4.2 K is the broadest yet seen, 19 mm/sec, with little structure evident. The linewidth decreases with small applied fields, becoming some 30% narrower. Analysis of small field data in terms of slow‐relaxation‐limit parameters yields an effective axial electronic‐splitting parameter D≈ + 30 mK at 4.2 K. Spectra obtained in large external fields yield hyperfine fields H_eff = − 326 (3) and − 330 (3) kOe for the 1.2% and 9.2% samples, respectively. These values are in good agreement with H_eff = − 324 (3) kOe obtained with Eu(NH₃)₆, quite similar to ESR results obtained in dilute Eu–NH₃ solutions, and indicate the lack of conduction‐electron‐polarization effects on the hyperfine fields. Intermediate field spectra are analyzed in terms of relaxation. The Eu(NH₃)₆ rates and the rates obtained in the dilute samples at small external fields are consistent with an f^1/2 relaxation rate, where f is the Eu fractional concentration. For larger fields, the rate in the dilute samples slows as a function of applied field divided by the temperature. This is qualitatively explained by ionic line narrowing, with concomitant loss of ionic transitions due to energy conservation requirements. The data for the 1.2% Eu sample in high fields are also used to estimate an upper limit of Korringa relaxation to the conduction band, yielding a result considerably smaller than expected for Eu²⁺ in conductors.